Support MathJax conditionally

Added conditional MJ support with rendering from cdn.mathjax.org.

- MathJax script included in "head" tag of HTML **iff** "mj" frontmatter tag is "true" (without quotes) as in

```
---
title: something
mj: true

---
```

- MathJax support is undetectable (I think) in posts without the mj tag.
- Added (to-be-trashed) example post to demo. 
- Both inline and display math are supported and determined by the markup generator.

Caveat: the diff shows that I killed some extra whitespace, sorry :-(